Lithographic plate

ABSTRACT

A paper lithographic plate which can be imaged in an automaticcopying machine.

Unlted States Patent 11 1 3,559,577

{72] Inventor Burl K, Sagawa [50] Field Of Search 101/460, Minneapolis. Minn. 461 462, 463 467; 250/65.1

{21] Appl No. 774,513

l 23 Filed No 8 1968 [56] References Clted [45] Patented Feb. 2, 1971 UNITED STATES PATENTS I 1 Assignee Minnesota Mining and Manufacturing 2.713.822 7 1955 Newman 101 /460 q l y 3.168364 2/1965 Brandl et a1. 101/467 5mm Paul: 3,299,807 1/1967 Evensen 101/467 3 corporaiwn of Delaware 3,376,162 4/1968 Spencer et a1. 101/467X Primary Examiner-David Klein [54] LITHOGRAPHIC PLATE Alt0rne vKinney, Alexander, Sell, Steldt & Delahunt 6 Claims, N0 Drawings [521 US. Cl 101/460, 250/65; 101/467 [51] Int. Cl B4ln 1/00, ABSTRACT: A paper lithographic plate which can be imaged B4 1 n 3/00 in an automatic-copying machine.

the reflex exposed portion into position against the receptor.

and then retracts to place a next adjacent portion in position for the next exposure. Suitable intermediate and receptor sheet materials are described in U.S. Pat. No. 3.094.417.

During the heating step and the intermediate is pressed into firm contact with the receptor sheet and is lightly bonded thereto. On retraction of the intermediate, the receptor sheet remains attached until the intermediate passes around a small diameter guide roll, at which point the bond between the flexible tautly held intermediate and the somewhat less flexible receptor is destroyed and the receptor is ejected from the machine.

Both the receptor and the intermediate are provided with surface coatings comprising resinous or polymeric binders, these coatings being responsible for the easily broken bond established between the two sheets under heat and pressure. Typical polymers include ethyl cellulose or cellulose acetate propionate for the intermediate and poly-t-butylmethacrylate for the receptor, as indicated in said U.S. Pat. No. 3,094,417. The intermediate coating may additionally comprise 4- methoxy-l-naphthol and erythrosin, whereas a corresponding receptor or copy sheet may contain silver behenate and phthalazinone as image-forming reactant and image toner respectively.

The same intermediate is found to produce excellent ink receptive images on certain normally hydrophilic lithographic plates when similarly reflex exposed and heated. However the necessarily hydrophilic coating of the lithographic plate fails to bond to the intermediate coating, and retraction of the intermediate therefore fails to eject the imaged plate from the machine.

It has now been found possible to obtain fully adequate but still easily overcome bonding between intermediate and lithographic plate, and without affecting the hydrophilic printing surface, by applying, to the coated margins of the plate, narrow strips of heat sensitive-bonding material. In a preferred form the bonding material is both polymeric, hydrophilic and water-soluble. although oleophilic water-insoluble polymers have also been used.

Preferably the bonding material is applied along a narrow full margin of the trailing edge of the lithographic plate. After passing through the copying machine, the imaged plate is placed on the lithographic press with the coated marginal portion beneath the clamp, so that the strip of bonding material is completely covered.

A continuous narrow coating is most easily applied, but discontinuous or skip coatings are also effective. Thus a pattern of equally spaced dots or dashes of the bonding material may be used; or the material may be coated in a number of parallel narrow stripes, either straight or wavy.

Application of the coating along one or both side edges likewise results in effective discharge of the plate from the copying machine. The oleophilic-bonding materials are ink receptive and if applied in such locations would cause undesirable marginal inking of the copy unless first removed; hydrophilic binders avoid this difficulty and are particularly preferred when application along the side margins is to be practiced.

The bonding material must be so distributed as to result in smooth discharge of the lithoplate without wrinkling or cockling. Ordinarily a marginal stripe or pattern across the trailing edge is adequate; but for thinner or somewhat flimsy plates, some additional side margin coatings may be helpful. It is also possible to place small dots of bonding material at regularly spaced points over the entire plate surface. provided any resulting inability to obtain inking of the plate at those spots does not detract unduly from the appearance or usefulness of the resulting copies.

The total area and the thickness of the bond coatings are ad justed to correspond with the structure of the plate and intermediate and with the bonding properties of the particular ad hesive material. More adherent materials need be applied only over minimum areas and in very thin layers. whereas very weakly adherent materials will cover much larger areas of the plate surface, or to a greater thickness, or both.

The bonding materials are preferably selected to be substantially nontacky under normal storage conditions, so that liners and slipsheets are not required and the plates may be stacked or boxed without blocking or adhering together. At the elevated temperature generated in the copying apparatus, and which may for example be in the neighborhood of 225 F. to 275 F., the material softens and becomes sufficiently tacky to form a weak but adequately slip resistant bond to the intermediate. The bond is easily broken, as the two sheets are carried over the discharge roller. by the force exerted by the rather stiff plate. thereby permitting separation of the plate from the intermediate.

Polyvinylpyrrolidone is a water-soluble. normally nontacky, heat softenable polymer which is easily applied from aqueous solution and forms an innocuous hydrophilic coating which is rapidly softened and rendered adherent at 225275 F. to form a fully effective but easily disrupted bond between the lithographic plate and the intermediate, and is presently preferred in the practice of the invention. The water-soluble normally nontacky copolymer of the invention of methylvinyl ether and maleic anhydride is also useful. Another useful water-soluble and hydrophilic temporary bonding material is a mixture of equal parts by weight of polyvinyl alcohol and zinc chloride dihydrate, the water of hydration being released at the elevated temperature to soften and tackify the polymer. Polyvinyl acetate, poly-t-butylmethacrylate, and Piccolyte S- terpene resin are examples of nonwater-soluble and oleophilic bonding materials which may be applied either from solution in an organic solvent such as acetone or from a dilute aqueous emulsion, and which form an ink receptive coating but which may serve as a slip-resisting binder.

EXAMPLE A lithographic plate comprising a heavy 73 lb. high wet strength paper having a hydrophilic surface coating of about 10 lbs/ream of a composition container containing 50 parts by weight of clay, 5 parts of polyvinyl alcohol. 0.5 parts of glycerine, 1 part of dimethylolurea and 1.5 parts of potassium iodide, and cured for 1 hour at 60 C., is fed into a Model 209" copying machine as described in U.S. Pat. No. 3,318,219, in contact with a previously exposed portion of an intermediate made with 4-methoxy-l-napthol and a photosensitizing dye in an ethyl cellulose binder. Heating at 225275 F. causes the formation of a corresponding faint visible image on the plate surface but the plate remains on the heating platen when the intermediate is retracted, and must be removed by hand.

Another lithoplate master of identical construction is coated along one end with a marginal stripe one-fourth inch in width and extending across the full width of the sheet, using as the coating composition an aqueous l0 percent solution of polyvinyl pyrrolidone applied at a coating weight, after drying, of about one-half gram per sq. ft. The sheet is fed into the copying machine with the coated margin at the trailing edge and is heated against another exposed portion of the intermediate. A visible and ink receptive image is formed, and the coated margin is adhered to the intermediate with sufficient bond to hold the two together as the heating platen is lowered and the intermediate is retracted. The lithoplate master is thereby ejected from the machine. The bond between the two webs is broken as the intermediate changes direction around the guide roller. and the plate is freed to drop into the recovery chute of the machine.

Applied in the area and at the thickness indicated. polyvinylpyrrolidone supplies sufficient bonding strength to cause smooth positive ejection of the plate from the machine. including both effective removal of the plate from the machine and effective separation of the plate from the intermediate. At the same coating weight. doubling the area of the stripe. e.g. by increasing the width to one-half inch. produces equally effective plate ejection. whereas trebling the stripe width makes separation of the plate and intermediate uncertain and requires careful inspection during this stage of the process. Either one-half or three-fourths inch stripe width is fully effective at coating weights about half that of the example, whereas at the lighter weight and in lesser widths. e.g. of one-fourth inch. the bond obtained is frequently insufficient to cause removal of the plate from the machine Bonding ability may be determined with sufficient accuracy for the purposes of this invention by uniformly coating the material on the plate surface, pressing strips of the coated plate together in face-to-face contact under moderate pressure with a heated roller, and then separating the strips at room temperature by peeling one from the other with each at approximately a right angle to the remainder of the combined strip. while measuring the pull required. Coating weights and strip widths are varied as required so as to obtain peel values within the approximate range of l0 to I40 grams. or preferably between about 20 and about l00 grams.

l claim:

I. A lithographic plate suitable for imaging in an automaticcopying machine as herein identified. said plate comprising a paper base and a hydrophilic coating and including along at least one edge of the coated surface a further narrow marginal coating of a normally nontacky heat activatible low strength bonding material. said marginal coating having a peel value of from about lOto about l40 grams. g

2. The lithographic plate of claim 1 wherein said bonding material is oleophilic.

3. The lithographic plate of claim 1 wherein said bonding material comprises a hydrophilic heat softenablc polymer.

4. The lithographic plate of claim 1 wherein said bonding material comprises a water-soluble polymer and a hydrated salt.

5. The lithographic plate of claim 1 wherein said narrow marginal coating is along the trailing end of said plate.

6. The lithographic plate of claim I wherein said narrow marginal coating is along both side edges of said plate. 

2. The lithographic plate of claim 1 wherein said bonding material is oleophilic.
 3. The lithographic plate of claim 1 wherein said bonding material comprises a hydrophilic heat softenable polymer.
 4. The lithographic plate of claim 1 wherein said bonding material comprises a water-soluble polymer and a hydrated salt.
 5. The lithographic plate of claim 1 wherein said narrow marginal coating is along the trailing end of said plate.
 6. The lithographic plate of claim 1 wherein said narrow marginal coating is along both side edges of said plate. 